Presumably coniferous plantations. These will cause a lot of shade, and change your microclimate, for example blocking the wind from the direction of the plantings, and perhaps creating frost pockets, depending on the lie of the land and something of a rain shadow, perhaps snow drifting.
Were it me, I would want to be sure that the conifers were planted well back, with deciduous native trees closer to your property. They are supposed to do this next to roads now, but you may have to consult your solicitor to make sure this happens in your case around your land. Have a close look at your land and imagine it with tall trees all around. Then you will know specific things you are not happy about, rather than just 'the trees will be too close'.
You would want to insist on good fencing, and if you can, get it at their expense, to ensure the safety of your livestock. Very difficult extracting livestock and dogs from plantations!
Not knowing the layout of your holding, in general I would want my ground to be open to the south, and perhaps sheltered from the prevailing wind, usually SW. That would mean buy the land to the south, and let them put trees to the SW and maybe NE if you suffer from snow.
Of course until the land has sold you won't know any purchaser's actual intentions.